CP studies open windows on new physics

Charge-parity (CP) violation parameters in tree-dominated b → c c s quark transitions are a powerful probe of physics beyond the Standard Model (SM). When B⁰_(s) and B⁰_(s) mesons decay through these transitions to the same final-state particles, an interference between mixing and decay amplitudes occurs, making these processes particularly sensitive to CP violation.

In the SM, B⁰_(s) – B⁰_(s) mixing is possible because the flavour eigenstates are not the (physical) mass eigenstates: a neutral B meson, once produced, evolves as a quantum superposition of B⁰_(s) and B⁰_(s) states. Due to this time-dependent mixing amplitude, an interference between mixing and decay amplitudes can lead to an observable time-dependent CP asymmetry in the decay rates. It was through the observation of this phenomenon in the “golden mode” B⁰ → J/ψ K⁰_s that, in 2001, the BaBar and Belle collaborations reported the first unequivocal evidence for CP violation in B decays, for which Kobayashi and Maskawa were awarded the 2008 Nobel Prize in Physics.

As the 3 × 3 Cabibbo–Kobayashi–Maskawa (CKM) matrix that describes quark mixing in the SM is expected to be unitary, it leads to relations among its complex elements. These can be represented as triangles in a complex plane, all of them with the same area (which is a measure of the amount of CP violation in the SM). The most famous of them, the so-called unitary triangle, has sides of roughly the same size and internal angles denoted as α, β and γ. Since individually none of the CKM parameters are predicted by theory, the search for new physics relies on over-constraining them by looking for any hint of internal inconsistency. For that, precision is the key.

LHCb has become a major actor in precision studies of CP violation

Having analysed the full proton–proton collision data set with 13 TeV, and adding it to previous measurements at 7 and 8 TeV, LHCb recently brought the CP-violating parameters in B⁰ → J/ψ K⁰_s  and in another golden channel, B⁰_s → J/ψ K⁺K^–, to a new level of precision. These parameters (sin2β and φ_s, respectively) are predicted with high accuracy through global CKM fits and, given their clean experimental signatures, are paramount for new-physics searches. The measured time-dependent CP asymmetry of B⁰ and B⁰ decay rates is shown in figure 1 with the resulting amplitude proportional to sin2β. Similarly, the update of the B⁰_s → J/ψ K⁺K^– analysis with the 13 TeV data resulted in the world’s most precise φ_s measurement. Both angles agree with SM expectations and with previous measurements.

These legacy results for sin2β and φ_s from the first LHC runs represent a new milestone in LHCb’s hunt for physics beyond the SM. Along with the world-leading determination of γ (with a current precision of less than four degrees), and the discovery of CP violation in charm in 2019, LHCb has fulfilled and exceeded its own goals of more than a decade ago, becoming the major actor in precision studies of CP violation. LHCb is taking data with a brand new detector at larger interaction rates than before, boosting the experimental sensitivity and tightening the grip around the Standard Model.